Hello C Nash - when posting, there is an icon to the right of the smiley face that when hovered over says, "Image", when you click on that you will put the link to the image you want to upload.
The image would need to be posted online somewhere, and then you put the URL for the image when you click on that icon. Here's an example image URL: http://www.rvusa.com/images/rvusa-logo.png
I hope that helps, but let me know if it doesn't. : )